Abstract
Abstract We present observations of main-belt comet (MBC) 259P/Garradd from 4 months prior to its 2017 perihelion passage to 5 months after perihelion using the Gemini North and South telescopes. The object was confirmed to be active during this period, placing it among seven MBCs confirmed to have recurrent activity. We find an average net pre-perihelion dust production rate for 259P in 2017 of M ̇ d = ( 4.6 ± 0.2 ) kg s−1 (assuming grain densities of ρ = 2500 kg m−3 and a mean effective particle size of a ¯ d = 2 mm) and a best-fit start date of detectable activity of 2017 April 22 ± 1, when the object was at a heliocentric distance of r h = 1.96 ∓ 0.03 au and a true anomaly of ν = 313.°9 ± 0.°4. We estimate the effective active fraction of 259P’s surface area to be from f act ∼ 7 × 10−3 to f act ∼ 6 × 10−2 (corresponding to effective active areas of A act ∼ 8 × 103 m2 to A act ∼ 7 × 104 m2) at the start of its 2017 active period. A comparison of estimated total dust masses measured for 259P in 2008 and 2017 shows no evidence of changes in activity strength between the two active apparitions. The heliocentric distance of 259P’s activity onset point is much smaller than those of other MBCs, suggesting that its ice reservoirs may be located at greater depths than on MBCs farther from the Sun, increasing the time needed for a solar-irradiation-driven thermal wave to reach subsurface ice. We suggest that deeper ice on 259P could be a result of more rapid ice depletion caused by the object’s closer proximity to the Sun compared to other MBCs.
Highlights
Comet 259P/Garradd was discovered on 2008 September 2 (Garradd et al 2008) when it was at a heliocentric distance of rh = 1.817 au, a geocentric distance of ∆ = 0.938 au, and a true anomaly of ν = 18.3◦
We find an average net pre-perihelion dust production rate for 259P in 2017 of Md = (4.6 ± 0.2) kg s−1 and a best-fit start date of detectable activity of 2017 April 22 ± 1, when the object was at a heliocentric distance of rh = (1.96 ∓ 0.03) au and a true anomaly of ν = (313.9 ± 0.4)◦
The heliocentric distance of 259P’s activity onset point is much smaller than those of other MBCs, suggesting that its ice reservoirs may be located at greater depths than on MBCs farther from the Sun, increasing the time needed for a solar irradiation-driven thermal wave to reach subsurface ice
Summary
MBCs orbit in the main asteroid belt, yet exhibit comet-like activity consistent with the sublimation of volatile material (Hsieh & Jewitt 2006; Snodgrass et al 2017). MacLennan & Hsieh (2012) measured 259P’s R-band absolute magnitude to be HR = (19.71 ± 0.05) mag, corresponding to an effective nucleus radius of re = (0.30 ± 0.02) km (assuming an R-band albedo of pR = 0.05) They found a slope parameter value of GR = −0.08 ± 0.05, and an estimated photometric range (peak to trough) due to rotational variations of ∆m ∼ 0.6 mag, an actual rotation period could not be definitively identified.
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